1. Field of the Invention
This invention relates to a filler for ink jet recording paper. More specifically, it relates to an amorphous silica filler capable of giving recording paper on which ink jet printing can produce dots having excellent circularity and clear and vivid colors.
2. Description of the Prior Art
Ink jet recording has various advantages such as reduced noises, high-speed recording and the ease of multi-color printing, and has found use in various printers and facsimile devices. Ordinary high-quality paper or coated paper is difficult to use as recording sheets in this application because of its insufficient properties. Recording sheets used for this purpose should meet various requirements. For example, ink droplets adhering to a sheet surface should be absorbed rapidly within the sheet. Spreading or blurring of ink droplets on the sheet surface should be suppressed. Clear images having a significant density should be produced on the sheet surface, and these images should have excellent fastness characteristics.
It has been proposed to coat or include various inorganic solid substances, optionally together with binders, on a paper surface or within the paper in order to impart these characteristics to the surface of the paper substrate. For example, it has already been known to use synthetic silica and/or its salt (Japanese Laid-Open Patent Publication No. 157786/1982), apply a weak acid salt or an oxide for a divalent metal such as magnesium or zinc as a coated layer (Japanese Laid-Open Patent Publication No. 94491/1983), include natural zeolite, synthetic zeolite, diatomaceous earth, synthetic mica, etc., in a coated layer (Japanese Laid-Open Patent Publication No. 68292/1984), and use clay, talc, calcium carbonate, kaolin, acid terra abra, activated terra abra, etc. as a white pigment forming an ink absorbing layer (Japanese Laid-Open Patent Publications Nos. 89391/1983 and 95188/1984).
It is never sufficient, however, for fillers in ink jet recording sheets, to merely absorb ink. A certain fixed balance is required between absorption of the entire ink or adsorption of water or a solvent and adsorption of a dye. For example, when ink is absorbed rapidly in the filler layer, spreading or blurring of ink drops will be prevented. But since the dye also permeates through the filler layer, the resulting image tends to lack clearness or vividness and a proper density. It will be understood therefore that it is by no means easy in ink jet recording to satisfy simultaneously the requirement of preventing spreading or blurring of ink droplets and the requirement of increasing the density and clearness or vividness of images.
Ink for ink jet printing is generally a composition comprising a dye dissolved in a medium containing water and a small amount of an organic solvent. Fillers heretofore used are not fully satisfactory in respect of a balance of adsorption of various components in the ink. For example, if the adsorption speed of the dye is too high, the dot diameter decreases, and white spots occur in the solid portion. If the adsorption speed of the dye is too low, dot circularity cannot be obtained.
As an improvement over these items, we previously proposed the use of amorphous silica having a median diameter, measured by the coulter counter method, for 2 to 15 .mu.m, an oil adsorption of at least 180 ml/100 g, a refractive index, measured by the solvent method, of at least 1,450, and a moisture absorption, measured when it is caused to absorb moisture for 200 hours at a relative humidity of 90% and a temperature of 25.degree. C., of at least 35% as a filler for ink jet recording paper (Japanese Patent Application No. 136308/1986).
The amorphous silica filler proposed in this prior application simultaneously satisfied the requirement of preventing spreading of ink droplets and blurring and the requirement of increasing the density and clearness of an image in ink jet recording, but has been found to have a problem still to be solved. This problem is that a dye dot formed by ink jetting on recording paper using this filler has an excellent color density or vividness in the early stage, but is liable to fade or discolor under light with the lapse of time.